Hinduism does not believe in sin or Hell the way the West does. In Hinduism everything is driven by karma (actions) and dharma (one's duty, path). Therefore we don't believe suicide will condemn a soul to Hell.
However, there is a reason we are here in our current incarnations. If we short-circuit that by committing suicide we may be subject to having to return to finish what we didn't finish. An example is that if I'm having problems in my family life or marriage, and I think the only way to solve them is to walk out on the family, that may be unwise karmically. It's very possible I am in the situation precisely because I walked away from my family in a previous life and caused them pain and hardship.
If someone is mentally ill and commits suicide that may be part of the soul's karma. Keep in mind karma IS NOT "paybacks are a b****", or reward for good deeds and punishment for bad deeds, or a direct one-to-one cause and effect. Karma is simply action and the fruits of action.
These are all speculations even on the part of the acharyas and gurus (teachers), saints, and rishis (sages, seers). No one has ever really shared details of previous lives. And there is a reason for that.
We have no right to ask when a sorrow comes, ‘Why did this happen to me?’ unless
we ask the same question for every joy that comes our way. - Lord Rāma to Lakshmana